This invention relates to tension leg moored structures and particularly, though not exclusively, to offshore wind turbine structures. Tension leg moored oil platforms are known, and it has been proposed to use tension leg moored structures for supporting wind turbines. Examples are disclosed in GB2365905, DE10101405, WO2008/122004, WO2009/064737 and WO2004/61302.
Such structures comprise a floating surface element moored to the sea floor by tethers. They allow operation in deeper waters (for example they are used at more than 200 meters in depth) than would be feasible using a rigid platform.
Particularly acute problems arise when seeking to stably anchor tall and narrow columnar upright floating structures such as wind turbines. In such structures, the wind turbine vertical shaft (supporting either a horizontal axle turbine mounted at its upper end or a vertical axle wind turbine around it) has a floating body centred around it, with (typically six or eight) radial outrigger arms at its lower end, beneath sea level. The outrigger arms are typically horizontal. The outer ends of the radial outrigger arms are interconnected by a horizontal ring of struts so as to provide rigidity to the outriggers against out-of-plane loading. Rising diagonally upwardly and inwardly from the outer ends of the radial outrigger arms are respective diagonal spars which meet the central vertical shaft. The outrigger structure thus provides a rigid upper connection platform from which a plurality of tethers (typically one coupled to each outrigger arm for eight or more outriggers; two, three or more per arm for fewer outriggers) run down to anchor points on the sea bed. Each diagonal spar thus lies on the hypotenuse of a right-angled triangle with a horizontal base provided by the outrigger arm and a vertical side provided by the centre axis of the wind turbine. The diagonal spars are maintained in tension by the downward force exerted by the tethers on the outrigger arms and the upward force exerted by buoyancy on the central body.
The tethers are below surface level. The tether top connection points define a surface centrally within which is the centroid of the forces acting on the tethers. When they are accurately aligned, the buoyancy of the floating body tensions all tethers equally. A change in depth of the surface structure varies the tension in the tethers equally. A horizontal force in the plane of the tether centroid increases the tension in all tethers equally. An overturning moment about the tether centroid on the outrigger structure increases the tension in some tethers and reduces it in others. If the tension in a tether is reduced to zero, it goes slack and tends to be subject to high dynamic snatch loads when re-tensioned.